Chains of the kind described above can have curved back surfaces for sliding contact a chain guide. An example of a known chain of this kind is described in United States Patent Application Publication No. 2008/0020882, published on Jan. 24, 2008.
As shown in FIGS. 4A and 4B of this application, a plate 510, which can be a plate of a chain in which all of the link plates are identical, has a curved back surface 514 for sliding contact with a chain guide (not shown). In such a chain, the area of contact between the back surface 514 and the chain guide increases over time, and wear loss of the chain increases. In this prior art chain, the shape of the back surface 514 in the widthwise direction of the chain is a straight line Ct5, as shown in FIG. 4B, and the shape of the back surface in the longitudinal direction is an arc-shaped curve Ce5 having a radius of curvature Re5, as shown in FIG. 4A.
The chain travels within a space in which lubricating oil comes into contact with all parts of the chain. Droplets of the oil adhere to the back surface 514 of each link plate 510 of the first and second links in regions thereof that do not come into contact with the chain guide. When the chain is in motion, air in the oil space acts on the oil droplets adhering to the back surfaces 514 of the link plates causing the oil to flow on the back surfaces.
When the back surface 514 is straight in the widthwise direction, as shown by straight line Ct5 in FIG. 4B, most of the oil on the back surface 514 flows in the longitudinal direction of the chain as indicated by arrows in FIG. 4C. Therefore, the time during which the oil is held on the back surface 514 is prolonged, and lubricating performance is favorable. That is, the back surfaces of the link plates exhibit good oil holdability, and friction between the back surface 514 and the chain guide is low.
On the other hand, in the case of a chain in which the back surfaces are straight in the widthwise direction, only a small amount of lubricating oil flows from the back surfaces 514, along the side surfaces 512 of the plates and through the narrow gaps between overlapping plates, to the pin holes 513, through which the connecting pins extend. Thus lubrication of the connecting pins and the pin holes becomes inadequate, and excessive wear between the pins and the inner surfaces 515 of the pin holes can result in elongation of the chain.
When the back surfaces of the link plates have a widthwise shape in the form of an arc-shaped curve Ct6, as shown in FIGS. 5A, 5B and 5C, and the radius of curvature Rt6 of curve Ct6 is smaller than the radius of curvature Re6 of the longitudinal curve of the link plate backs, the contact area, between the back surfaces 614 of the link plates 610 and the chain guide is reduced. In this case, droplets of oil adhering to the back surface 614 flow readily from the back surface 614 and along both sides 612 of the link plates as indicated by arrows in FIGS. 5A and 5C. Accordingly, flow of oil to the pin holes 613 from the back surface 614 increases, and wear of the connecting pins and the inner surfaces 615 of the pin holes is reduced and premature elongation of the chain is avoided.
However, the backs of the link plates in FIGS. 5A-5C have reduced oil holdability, and consequently, friction between the back surfaces 614 and the chain guide or guides is greater. Moreover, because of the small radius of the widthwise curvature of the link plates, the contact area between the link plates and the chain guide is low and contact pressure is high. Because of inadequate lubrication and high contact pressure, excessive wear of the back surfaces 614, and of the chain guide, can occur.
Wear elongation of the chain, and wear between the back surfaces of the link plates and the chain guide, degrade the durability of the chain and the chain guide. Accordingly, there is a need for a chain that exhibits reduced friction and wear and improved durability, and which also contributes to improvement in the durability of a chain guide.